a Democrat running for Senate in 2008 was great. In 2014? Not so much.

By Philip Bump June 19 ?

NPR has a new poll with data that might give the casual observer pause. "In the 12 states with competitive Senate races this fall, only 38 percent of likely voters said they approved of the way the president is handling his job," NPR's Mara Liasson reports. "An index of all national polls shows the president's approval rating about four percentage points higher nationwide."

There are two charts that make this point. This is what President Obama's approval looks like in the 12 battleground states (Alaska, Arkansas, Colorado, Georgia, Iowa, Kentucky, Louisiana, Michigan, Montana, New Hampshire, North Carolina and West Virginia) according to the new NPR poll.

That's a big difference. But, as Liasson points out, it's probably not a huge surprise, given that eight of the 12 states went for Mitt Romney in 2012. In other words, Obama has been less popular in those states for a while.

But there's another important number to consider here: Six. As in, these Senate races come six years after the incumbents were elected. In 2008. When Obama was elected president in a Democratic wave election.

Nine of the 12 states saw an increase in the percentage of vote for the Democratic candidate between 2004 and 2008. Eight of the nine then saw Democratic vote decrease in 2012.

That Democratic bump was significant in a lot of down-ballot races, and likely helped in some of these Senate races, too. That year, the average margin of victory in those 12 races was 19 percent, which is huge. But take out the races involving essentially uncontested or retiring senators -- including Sen. Mark Pryor (D-Ark.), who won by almost 60 points -- and the spread was about 6 percent. In Georgia, Republican Sen. Saxby Chambliss (who is retiring this year) only won by 3 percentage points. In Alaska, Sen. Mark Begich (D) won by just over a point in a state that saw a 2.3 percentage point increase in Democratic turnout -- despite Sarah Palin being on the ballot.

Had Begich lost that race, or had Sen. Jeanne Shaheen (D-N.H.) lost hers (which she only won by 6.5 points in a state that gained 4 percent more Democratic votes), it's not clear that those states would still be battlegrounds today. (In Alaska's case, you can be nearly certain it would not have been a 2014 battleground.)

The most critical change in November, though, isn't between what happened in 2008 and what happened in 2012. It's what's likely to happen between 2012 and 2014. This is a midterm, taking place on generally hostile ground for Democrats. To Liasson's point, Obama got, on average, 44.7 percent of the vote in 2012 in these 12 states. That's an uphill climb, even when you're helped by a bump.

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